Prosthetic heart valves are used to replace damaged or diseased heart valves. In vertebrate animals, the heart is a hollow muscular organ having four pumping chambers: the left and right atria and the left and right ventricles, each provided with its own one-way valve. The natural heart valves are identified as the aortic, mitral (or bicuspid), tricuspid and pulmonary valves. Prosthetic heart valves can be used to replace any of these naturally occurring valves, although repair or replacement of the aortic or mitral valves is most common because they reside in the left side of the heart where pressures are the greatest.
Where replacement of a heart valve is indicated, the dysfunctional valve is typically replaced with either a mechanical valve, or a tissue valve. Tissue valves are often preferred over mechanical valves because they typically do not require long-term treatment with anticoagulants. The most common tissue valves are constructed with whole porcine (pig) valves, or with separate leaflets cut from bovine (cow) pericardium. Also, synthetic materials such as molded polymers have been proposed as substitutes for natural tissue. Although so-called stentless valves comprising a section of porcine aorta along with the valve fully intact are available, the most widely used valves include some form of stent or structural support for the leaflets. Natural tissue valves have a proven track record, but the manufacture thereof suffers from reduced yield of the tissue because of flaws discovered during the inspection process. For example, thickness variations in pericardial tissue renders some pieces unfit for leaflet formation.
A conventional heart valve replacement surgery involves accessing the heart in the patient's thoracic cavity through an incision in the chest. For example, a median sternotomy requires cutting through the sternum and forcing the two opposing halves of the rib cage to be spread apart, allowing access to the thoracic cavity and heart within. The patient is then placed on cardiopulmonary bypass which involves stopping the heart to permit access to the internal chambers. Such open heart surgery is particularly invasive and involves a lengthy and difficult recovery period.
Some attempts have been made to enable less traumatic delivery and implantation of prosthetic heart valves. For instance, U.S. Pat. No. 4,056,854 to Boretos discloses a radially collapsible heart valve secured to a circular spring stent that can be compressed for delivery and expanded for securing in a valve position. Also, U.S. Pat. No. 4,994,077 to Dobbin describes a disk-shaped heart valve that is connected to a radially collapsible stent for minimally invasive implantation.
Recently, a great amount of research has been done to reduce the trauma and risk associated with conventional open heart valve replacement surgery. In particular, the field of minimally invasive surgery (MIS) has exploded since the early to mid-1990s, with devices now being proposed to enable valve replacements without opening the chest cavity. MIS heart valve replacement surgery still typically requires bypass, but the excision of the native valve (in cases where the native valve is removed) and implantation of the prosthetic valve are accomplished via elongated tubes or cannulas. Endoscopes and other such visualization techniques can also be used to assist implantation.
Some examples of more recent MIS heart valves are shown in U.S. Pat. No. 5,411,552 to Anderson, et al., U.S. Pat. No. 5,980,570 to Simpson, U.S. Pat. No. 5,984,959 to Robertson, et al., PCT Publication No. 00/047139 to Garrison, et al., and PCT Publication No. WO 99/334142 to Vesely. Although these and other such devices provide various ways for collapsing, delivering, and then expanding a “heart valve” per se, none of them disclose an optimum structure. For instance, the publication to Vesely shows a tissue leaflet structure of the prior art in FIG. 1, and an expandable inner frame of the invention having stent posts in FIGS. 3A–3C. The leaflets are “mounted to the stent posts 22 in a manner similar to that shown in FIG. 5.” Such general disclosures as in Vesely stop short of explaining how to construct a prosthetic valve in a manner that maximizes long-term efficacy. In particular, the means of attaching the leaflets to the MIS stent is critical to ensure the integrity and durability of the valve once implanted.
Another problem with MIS valves of the prior art is their relatively large radial dimension during implantation. Most of these valves utilize one or more radially-expanding stents coupled to a biological valve, and the assembly must be compressed radially and then passed through the lumen of a large bore catheter. Reducing the radial profile of the constricted valve via radial compression is problematic and conflicts with the need for sufficient diameter of the valve in its expanded state to fit securely within an adult heart valve annulus.
Bioprosthetic tissue heart valves have proved particularly successful and durable, and substantially eliminate the need for long-term treatment with anticoagulants. Unfortunately, the use of bioprosthetic tissue in minimally invasive heart valve presents a number of challenges. First, minimally invasive heart valves are most effective if they are compressible into a small profile for delivery and then expandable at the site of implantation. Attachment of the bioprosthetic tissue to the structural component of the valve therefore must be able to withstand the valve compression and expansion. In addition, the xenograft valve or tissue leaflets are likely to be susceptible to damage by folding and pinching during valve compression. The potential for pinching the bioprosthetic tissue is particularly acute when the valve is compressed into a very small profile.
Despite some advances in heart valve design, and particularly MIS valve design, there remains a need for improved tissue characteristics and for a minimally invasive heart valve that can be compressed and expanded without damage to the flexible tissue leaflets.